1. Field of the Invention
The present invention relates to a dynamic damper which is attached to an inner peripheral space of a hollow rotating shaft such as a propeller shaft of a motor vehicle or the like, and suppresses a vibration and a noise which are generated in the hollow rotating shaft.
2. Description of the Conventional Art
A typical prior art of a dynamic damper, which is attached in an inner peripheral space of a propeller shaft corresponding to a hollow rotating shaft transmitting driving force output from an engine of a motor vehicle via a transmission to rear wheels and suppressing a vibration and a noise generated in this propeller shaft, is disclosed in Japanese Unexamined Patent Publication No. 2007-177830.
FIG. 9 is a sectional perspective view of an installed state and shows the same kind of conventional dynamic damper as that described in Japanese Unexamined Patent Publication No. 2007-177830 the patent document 1 by cutting along a plane passing through an axis together with a part of a propeller shaft. In other words, a dynamic damper 50 is constructed by an elastic body 51 which is formed in an approximately cylindrical shape by a rubber-like elastic material (a rubber material or a synthetic resin material having a rubber-like elasticity) so as to be pressure inserted to an inner peripheral surface of a propeller shaft 60, and a mass body 52 which is concentrically vulcanization bonded in the elastic body 51 and is made of a metal, and an additional vibration system constructed by the elastic body 51 and the mass body 52 reduces a vibration and a noise of the propeller shaft 60, on the basis of a dynamic damping action of resonating by an inverse phase to an input vibration, in a predetermined vibration frequency range.
However, as for the conventional dynamic damper 50, since the mass body 52 is pinched from both sides in an axial direction by convex portions (not shown) formed in a metal mold so as to be held, at a time of integrally vulcanization forming the elastic body 51 with the mass body 52, it is hard to make an outer diameter of the elastic body 51, that is, an outer diameter of the dynamic damper 50 small, in accordance with necessity for securing a pinching space. Accordingly, such is hardly applicable to a propeller shaft having a small diameter.
Further, when a cleaning of the propeller shaft is carried out after installation to an inner periphery of the propeller shaft 60, and the propeller shaft is stored in an upright state, a cleaning fluid entered into an inner portion of the propeller shaft 60 stays without being discharged. Further, there is a structure in which a plurality of notches 51a are formed in an outer peripheral surface of the elastic body 51, in order to facilitate installation to the propeller shaft 60, and the notches 51a contribute to the discharge of the cleaning fluid entered into the inner portion of the propeller shaft 60. However, since the elastic body 51 is formed in such a shape as to have inner peripheral holes 51b at both sides in an axial direction of the mass body 52, in accordance with necessity of securing a spring property in an axially orthogonal direction in the elastic body 51, and necessity of holding the mass body 52 by a part of the metal mold at a time of forming, the cleaning fluid entered into the inner peripheral hole 51b can not be discharged by the notches 51a. 
It can be thought that a drain hole is provided in an axial core portion of the mass body 52, however, in this case, not only the number of processing steps is increased, but also the forming rubber material leaks into the drain hole from the portion where the mass body 52 is held by a part of the metal mold at a time of forming the elastic body 51 so as to clog the drain hole, so that there is a risk that productivity is lowered.